Subfloor/thinset question


Old 08-19-14, 02:17 PM
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Subfloor/thinset question

I am retiling a small, 23 Sq/ft half bathroom that was previously done very poorly with very little thin set and thin tiles that cracked.

The tiles came up very easily, but I'm having trouble getting all of the thin set off the of the plywood subfloor. I've gotten much of it off and have ground down the spots that are left to a very thin and smooth surface. Is this okay to apply the tiles to as long as I make sure there is nothing loose? I'm avoiding cement board because I don't want to raise the floor any more than the much thicker tiles I'll be installing will.

Any advice would be appreciated. Thank you.
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Old 08-19-14, 03:00 PM
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I don't want to raise the floor any more
Not a valid reason to skimp on proper prep for the subfloor.

The tiles came up very easily
Exactly what will happen again if you follow your current plan. Tile needs a sturdy and rigid floor for a successful installation. Tiling directly over plywood does not work because the wood is subjected to seasonal expansion and contraction. It breaks the bond with the thinset and you end up with the mess of a floor like the one you are trying to replace. Cement backer boards allow the ply some marginal room to move and keeps the tile in place and also serves as a proper bonding agent between the thinset and tile. When set in a troweled bed of thinset, the thinset also acts to remove any dips/valleys/voids in the subfloor that will cause movement in the tile above. You can also look into Ditra which is thinner than cement boards but also more pricey.
Old 08-20-14, 07:24 AM
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Additionally, what's the size, spacing and unsupported span of the floor joists and what is the composition of the current subfloor?

FWIW, I would remove the layer of plywood on which the tiles were set rather than try to remove the thinset from it.
Old 08-20-14, 07:58 AM
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For 23 sf using Ditra or a similar membrane (I think there are several brands?) would seen a reasonable approach. You might be able to buy that small amount from a tile setter or company in bulk as opposed to buying a whole roll at retail.
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